The present invention relates generally to radio frequency (RF) receivers and more particularly to an input protection circuit for a RF receiver for preventing damage to receiver components from high power level input signals.
Radio frequency (RF) receivers typically have sensitive front-end components such as mixers and preamplifiers that are susceptible to damage from high power level input signals. For example, an input mixer may be rated at 30 dBm which is equivalent to less than one watt of power. The output of power amplifiers in wireless communications base stations can have 30 watts of power going to the antenna. Measuring the output of the power amplifier using a measurement test instrument having an RF receiver requires the insertion of a attenuation pad between the power amplifier output port and the measurement instrument. The front end components of the measurement instrument will be destroyed without the use of the attenuation pad or some other way of protection circuit in front of the sensitive front end components.
One type of protection circuit is found in Advantest spectrum analyzers. The RF input to the spectrum analyzer is resistively coupled to an overload peak detector prior to being coupled to an input switch. The output of the peak detector is coupled to a relay control that causes the input switch to couple the signal through a resistive load to ground when the input signal exceeds one of two threshold levels. One level has a range of 0 dBm to +12 dBm with 0 dB of attenuation and a preamplifier connected to the input. The other level has a range of +20 dBm to +27 dBm with attenuation added into the circuit. A power limiter is connected before the preamplifier to limit the input power with no attenuation applied. The first mixer does not have a power limiter in place prior to its input. The power limiter in the lower threshold state can reflect power back into the detector which may cause an incorrect power detection and activation of the input switch.
Drawbacks to this particular protection circuit include the use of a peak detector that would have a tendency to be inaccurate for different peak to average ratio signals. Additionally, the peak detector is not isolated from the input signal which could be damaged if a high input power signal is accidentally applied to the input of the instrument. Further, only the preamplifier has a power limiter for protection and not the first mixer. Even though the input signal is attenuated prior to the mixer, a high input power signal accidentally applied to the input could destroy the mixer.
What is needed is an input protection circuit for an RF receiver that is independent of the receiver setting. The protection circuit should have a narrow threshold window that is not affected by limiter reflections or the peak to average ratio of the detected signals. Further, the protection circuit should be able to protect the sensitive front end components of the receiver from signals in excess of 50 watts.
Accordingly, the present invention is to an input protection circuit for a radio frequency receiver that has an input switch, preferably a latching relay, for selectively coupling a radio frequency (RF) input signal to the input of an isolation device. The isolation device has a first output coupled to a power limiter that receives a first portion of the RF signal. The RF signal is coupled through the power limiter to the radio frequency receiver. A second output of the isolation device is coupled to an RMS level detector that receives a second portion of the RF input signal. The RMS level detector generates an output level that is the same for radio frequency signals of equivalent power but differing peak-to-average power ratios. A comparator having a latch receives a reference signal and the output level from the RMS level detector and generates and latches an output signal when the output level is greater than the reference signal. The input switch receives the latched output signal and selectively couples the RF signal to the isolation device when the output level of the RMS level detector is less than the comparator reference signal and de-couples the radio frequency signal from the isolation device when the output level of the RMS level detector is greater than the comparator reference signal. A reset circuit is coupled to the comparator that unlatches the output signal from the comparator.
In the preferred embodiment of the invention, the isolation device is an broadband directional bridge and the RMS level detector is a broadband level detector. Alternately, the isolation device may be a narrow band directional coupler. The comparator latch is preferably implemented as a programmable logic device that includes a message generator that produces an overload condition exits message when the output level of the RMS level detector is greater than the comparator reference signal. The overload condition exists message is preferably a text message displayed on a display device. Alternately, the message may be an audio message generated from a speaker or a warning light indicator produced by an output drive signal. In the preferred embodiment, the radio frequency power limiter has a continuous wave radio frequency power rating of three watts. The latching relay has an armature coupled to receive the radio frequency signal, a first armature contact coupled to the isolation device and a second armature contact coupled to electrical ground or alternately to a resistive load.
In an alternate arrangement of the circuit elements of the input protection circuit for a radio frequency receiver, the input of the isolation device is coupled to receive the RF input signal. The RMS level detector is coupled to the first output of the isolation device and receives a first portion of the radio frequency input signal. The RMS level detector generates an output level that is the same for radio frequency signals of equivalent power but differing peak-to-average power ratios. The comparator receives a reference signal and the output level from the RMS level detector and generates an output signal when the output level is greater than the reference signal. The switch disposed after the isolation device and is coupled to receive a second portion of the radio frequency signal from the second output of the isolation device with the switch being driven by the comparator output signal to selectively couple the second portion of the radio frequency signal to the radio frequency receiver via a radio frequency power limiter when the output level of the RMS level detector is less than the comparator reference signal and de-couples the radio frequency signal from the radio frequency power limiter when the output level of the RMS level detector is greater than the comparator reference signal. The switch has an armature coupled to second output of the isolation device, a first armature contact coupled to the power limiter and a second armature contact coupled to electrical ground or alternately to a resistive load.
The objects, advantages and novel features of the present invention are apparent from the following detailed description when read in conjunction with appended claims and attached drawings.